It prevails that construction machines having a swing structure (e.g., hydraulic excavators) rotates a hydraulic motor by driving a hydraulic pump with an engine and by using hydraulic fluid delivered from the hydraulic pump, thereby driving the swing structure (as an inertial body). In recent years, however, in order to improve the fuel efficiency of the engine, lower the noise level, reduce exhaust gas emission, and so forth, there have been proposed hybrid construction machines in which the swing structure is driven by using both the hydraulic motor and an electric motor that performs the driving by use of electric energy supplied from an electrical storage device. In such hybrid construction machines, it is necessary to appropriately control the drive torque (generated by the shared work of the hydraulic motor and the electric motor) so that operators accustomed to the operation of conventional construction machines can operate the hybrid construction machine with no feeling of strangeness.
As a conventional technology designed for the purpose of enabling continuous and smooth drive control of the swing structure (as an inertial body) and efficient energy regeneration, there exists control means for a hybrid construction machine (see Patent Literature 1, for example). The control means calculates a torque command value for the swing electric motor (electric motor used for the swing driving) based on the differential pressure occurring between two ports of the swing hydraulic motor (hydraulic motor used for the swing driving) serving as a hydraulic fluid suction hole (inlet side) and a hydraulic fluid discharge hole (outlet side).